Vitamin D.sub.3 is commonly prepared, in commercial practice, by a process involving the photolysis of 7-dehydrocholesterol, to form an intermediate product previtamin D.sub.3, which is then heated to convert it to vitamin D.sub.3, thus: ##STR1## the group R in the above formula representing the branched alkyl chain of vitamin D.sub.3, which is well known and remains unchanged in the reactions. Vitamin D.sub.2 differs therefrom chemically only in minor respects relating to the nature of the branched alkyl group R. Ergosterol is an available natural material, while 7-DHC is commonly synthesized from cholesterol. The process as a whole is made more expensive by the relatively low yields of previtamin - D.sub.3 (pre-3) which may be obtained. Pre 3 is liable to undergo side photolysis reactions, to form lumisterol (L3) and tachysterol (T3). At least according to some of the published literature, the photolysis is often stopped at 30-40% conversion of the 7-DHC, when the concentration of Pre 3 in the mixture is at a maximum, the product mixture separated and recycled. Whilst the reactions of Pre 3 to form T3 or L3 are reversible, D.sub.3 cannot be formed directly from either T3 or L3. The reaction system may be represented as follows: ##STR2##
For a commercially successful and economic process, therefore, the system must be optically pumped so as to produce a maximum concentration of Pre 3 while minimizing the concentrations of L3, T3, and residual 7-DHC. In addition, it is of course desirable to minimize the energy costs for the irradiations.